Japanese Patent Application, Publication No. 2001-83397 discloses an optical apparatus that includes an autofocus (hereinafter “AF” for short) lens, a first pulse encoder that detects a driving speed and driving amount of the AF lens, and a second pulse encoder that detects a driving speed of an AF actuator, wherein the optical apparatus controls the speeds based on an output from the second encoder when the AF actuator runs and accelerates, and based on an output from the first pulse encoder when the AF actuator decelerates and stops.
Such an optical apparatus includes a mechanism that transmits a driving force from the AF actuator to the AF lens, and the mechanism has a complicated mechanical structure composed of a gear, a focus ring, a manual focus (“MF”) ring, etc. This mechanical structure would generate frictions, and the speed detection at one location causes the AF motor to stop or suddenly accelerate in response to a user's manual manipulation. A structure disclosed in the above reference rectifies this shortcoming, and stabilizes the AF action.
Current cameras usually have the AF function, and are required to provide a more precise AF and a shorter focusing time upon the subject.
The precise AF requires a precise encoder used to detect a position of a focus lens.
Nevertheless, the actuator actually drives the focus lens, and a transmission mechanism transmits a driving force applied by the actuator to the focus lens.
The transmission mechanism is configured to connect plural gears and generates backlashes and deflections, causing a difference between the actual driving speed of the actuator and the moving speed of the focus lens.
Therefore, the speed detecting location by the encoder greatly affects a stop position and driving speed of the focus lens. In other words, even though the encoder becomes precise, the accuracy of the stop position does not improve if the driving speed of the focus lens is unstable. A detailed description will now be given of this problem.
FIG. 10 shows a structure of a focus unit driving mechanism that uses a conventional actuator. The driving force of an actuator 130 is transmitted to a gear part 151a in a fixed-position rotating ring 151 via plural gears 131 to 134, and drives the fixed-position rotating ring 151. This force drives a focus unit 102 that is helicoidally connected to the fixed-position rotating ring 151; a focus lens (not shown) in the focus unit 102 moves and is focused upon the subject.
The encoder includes pulse plates 143 and 121, and photointerrupters 104 and 105. The pulse plate has holes at regular pitches in a rotational direction. The rotational amount and rotational speed are detectable by reading the light transmitting state and the light shielding state of the pulse plate using the photointerrupters.
The pulse plate 143 and 121 need a smaller hole pitch for improved position detecting accuracy of the focus unit 102. When the stop position of the focus unit 102 and the speed of the actuator 130 are controlled while the encoder is arranged at a position A in FIG. 10, mechanical looseness, deflections, deformations, etc. may slightly retard a transmission of the driving force of the actuator 130 to the focus unit 102.
In other words, when an output of the pulse plate 143 is detected and controlled at the position A, the rotational amount and rotational speed of the actuator 130 can be precisely detected. However, the detected rotational speed may not correspond to the moving speed of the focus unit 102. In this case, the actual focus unit 102's stop position scatters even when the actuator 130 is controlled to stop for each predetermined driving amount.
On the other hand, when the stop position of the focus unit 102 and the speed of the actuator 130 are controlled while this encoder is arranged at a position B in FIG. 10, the speed control is maintained without influence of the mechanical looseness, deflections, deformations, etc. until the driving force of the actuator 130 transmits to the focus unit.
However, when the actuator 130 is activated with large mechanical looseness (which is caused, for example, by backlashes among gears when the actuator is driven in a direction reverse to the last driving direction), the focus unit 102 does not move due to the mechanical looseness until the looseness is removed. The encoder starts detection after the focus unit starts moving.
In other words, due to no output at all from the encoder just after the actuator 130 is activated, the control accelerates the speed of the actuator 130. When the mechanical looseness is removed, the accelerated driving of the actuator 130 transmits to the focus unit 102 and might cause the moving speed of the focus unit 102 to exceed the predetermined target speed.
Conceivably, in an attempt for a slight driving, for example, this phenomenon would cause the focus unit 102's inertial force to move the focus unit 102 beyond the predetermined driving amount, even when the actuator 130 stops after the focus unit 102 moves by the predetermined amount. In other words, the precise control over the stop position of the focus unit 102 is not available.
One preferable solution for this problem is to arrange photointerrupters at both the positions A and B. However, the above control method that utilizes the speed detections at two positions requires the information of the current driving direction to be stored in a memory, for example, so that the control method can determine whether or not there is looseness in the next driving. In addition, switching of the speed detecting encoders between the positions A and B for the acceleration and the deceleration makes the control complicated disadvantageously.
Moreover, as to the detection accuracy of the driving amount, the accuracy of the output values relating to the driving amount may be worse than the usual one due to a hole pitch difference between the pulse plate 143 at the position A and the pulse plate 121 at the position B, and a relationship between a reducing ratio from the actuator 130 to the pulse plates 143 and 121 and a reducing ratio from the actuator 130 to the focus unit 102. It is therefore not necessarily the best method to switch the speed control based on the output from the encoder at the position A and the speed control based on the output from the encoder at the position B.
In addition, the detection accuracy of the driving amount would differ between DC and vibrating motors applicable to the actuator. This is because the DC motor generally rotates at a high speed with a low torque whereas the vibrating motor rotates at a low speed with a high torque. Therefore, for the same moving speed of the focus unit, use of the DC motor would require a larger reducing ratio and more gears in FIG. 10.
A description will be given of the stop position accuracy, for example, where the DC motor and the vibrating motor are used for the actuators for driving two lenses having approximately the same mechanical structure in driving force and moving speed of the focus unit. While these motors are connected to the pulse plates having the same hole pitch, the number of output pulses of the encoder is detected relative to the rotational amount of the fixed-position rotating ring. As a result, the lens that uses the DC motor has more output pulses due to the above relationship of the reducing ratio.
According to this structure, use of the DC motor would be able to detect a position of the focus unit with higher precision. Use of the vibrating motor as the actuator would provide a worse position detection accuracy than the DC motor due to the above relationship of the reducing ratio.
From this relationship, in order for the lens that uses the vibrating motor to obtain the same position detection accuracy as the lens that uses the DC motor, it is necessary to provide a reduction mechanism between the vibrating motor and the focus lens, and gears that increase the motor's speed between the vibrating motor and the pulse plate.
While a conceivable method might be to associate the speed-increased final gear with the pulse plate for improved stop position accuracy, it would hardly actually improve the position detecting accuracy of the focus unit, since the mechanical looseness increases between the vibrating motor and the pulse plate in addition to the increasing mechanical looseness between the vibrating motor and the focus unit.
Apparently, it is preferable to use two position detecting mechanisms to detect a position of the lens that uses the vibrating motor, i.e., one that omits the speed increasing mechanism by gears and the other that detects a position of the focus unit. In particular, the position detecting mechanism for the focus unit preferably uses a pulse plate having a smaller hole pitch for improved accuracy.
It is thus preferable to detect the position and speed at both the positions A and B in FIG. 10. However, the position detecting accuracy at the position B may be better due to the actuator and the mechanical structure.
In view of these mechanical structures, the speed control should select one of detection systems at two points based on the past and current states. This selection necessity complicates the control and requires a high-performance (or expensive) microcomputer.